As will be apparent from the aforementioned copending application and related work in the field (see the patents listed above), it is known to provide a high-pressure blast furnace with a gas-cleaning plant which obtains a maximum of useful energy from the exhaust gas, controls effectively the back pressure of the blast furnace, and prevents pollution of the atmosphere.
Basically, such an apparatus can comprise, in addition to the blast furnace and the usual gas locks, gates or bells affording withdrawal of the exhaust gas in large volume while maintaining the back pressure, at least of the following important components of the gas-cleaning plant:
(a) a coarse dust or particle separator; PA1 (b) a washing or scrubbing tower; PA1 (c) an expansion-turbine unit; and PA1 (d) a scrubbing water separator.
The first of these units, namely, the coarse dust or particle separator may be a cyclone from which particulates settle or are otherwise discharged from the gas and can include dust-pocket or collector-type dust catchers using any conventional dust-recovery mode of operation.
From that coarse-particle separator, the gas is subjected to scrubbing in a wash tower or scrubbing tower which can be provided at an upper or upstream portion thereof with a multiplicity of turbulent spray nozzles or jets (swirl jets). In the lower end of the washing tower there is provided at least one Venturi element constituting, for example, a Venturi scrubber and serving at least in part to control the pressure differential across the Venturi passage to thereby regulate the back pressure in the blast furnace.
The exhaust gas which has discharged its particles in the dust collector and has been scrubbed and cooled in the scrubbing tower and the Venturi scrubber, is then generally admitted to an expansion turbine for the work-producing expansion of the gas which converts its potential energy into useful work. For example, the expansion turbine can operate an electric-power generator. The gas is then generally subjected to further processing to recover it heat value, i.e. can be recirculated to combustion chambers or metallurgical processes and the residual heat value of the components of the gas can be utilized.
The scrubbing-water unit recovers the wash water used in the scrubbing and washing tower and can include a settling and separating tank in which solids are separated from the liquid, the solids being in the form of a sludge from which the liquid is decanted.
Decanted liquid generally passes into a collecting tank and can be fed to a cooling installation, e.g. a force-draft cooling tower, before being supplied again to the scrubber.
The turbine installation is provided with a bypass so that, when the turbine is cut off, the scrubbed exhaust gas can be discharged from the system to the installation capable of recovering the heat value in the manner described.
The Venturi scrubber can be of the annular-gap washer type described in any of the aforementioned patents and usually comprises a Venturi duct in which, in the region of the most constricted portion of the duct, a frustoconical or other convergent or divergent body is displaceable for control of the pressure drop across the Venturi passage.
The term "heat value" is used herein to refer generally to the chemical energy of the gas when the latter is burned. The heat value may be due to the presence of oxidizable components such as hydrogen and carbon monoxide or to the presence of carbonaceous particles, hydrocarbons or even products of the reaction in the blast furnace which are not scrubbed out during the processing of the gas. Such heat value can be recovered by adding the gas to a fuel and subjecting the mixture to oxidation (combustion) or by using the recovering gas directly as a heating gas.
In the operation of such a plant, it is desirable that the enthalpy of the gas emerging from the blast furnace be as high as possible since a high enthalpy also corresponds to a high expansion-turbine efficiency. Thus the exhaust gas should be introduced into the expansion turbine unit at the highest possible temperature.
However, it will be recalled that the gas before introduction into the expansion-turbine unit is subjected to scrubbing, i.e. to an intensive washing with water which, if carried out to the most efficient degree, will result in substantially complete saturation of the gas with water. This, of course, reduces the heat value of the gas since it dilutes combustibles therein with water vapor.
Thus, in order to obtain a gas with high heat value it is necessary to limit the water content of the gas. This means that the gas temperature after treatment in the scrubbing tower should be relatively low.
From the foregoing it will be apparent that two countervailing interests are involved. On the one hand, effective operation of the turbine requires a maximum temperature after scrubbing while, on the other hand, desire to obtain maximum exploitation of the heat value of the same gas requires a lower temperature after scrubbing.
In conventional blast furnace gas-cleaning systems, therefore, the operation is a compromise and it is particularly undesirable that the discharged gas have a relatively high temperature and a low heat value when the expansion turbine is bypassed or cut off. In the latter case, one does not obtain the gas at an optimum heat value and does not make up this energy loss by recovering it at the expansion turbine.